1. Field of the Invention
The present invention relates to a focus monitoring method, a focus monitoring apparatus and a method of manufacturing a semiconductor device, and more particularly, to a focus monitoring method used for pattern formation of a semiconductor device, a focus monitoring apparatus, and a method of manufacturing a semiconductor device.
2. Description of the Background Art
In recent years, high integration and miniaturization of semiconductor integrated circuits have been remarkable. Accordingly, miniaturization of a circuit pattern formed on a semiconductor substrate (hereinafter simply referred to as a wafer) has rapidly been advanced.
Among others, a photolithography technique is widely recognized as a basic technique in pattern formation. Thus, up to the present date, various developments and improvements have been made to the photolithography technique. However, patterns have continuously been reduced in size, and also the demand for improvement of pattern resolution is becoming stronger.
The photolithography technique is a technique in which a pattern on a photomask (an original) is transferred onto a photoresist applied on a wafer and the photoresist having the transferred pattern is used to pattern an underlayer film to be etched.
The photoresist is subjected to a developing process at the time of transferring the pattern onto the photoresist. A type of a photoresist in which a portion of the photoresist exposed to the light is removed in the developing process is referred to as a positive type photoresist, whereas a type in which a portion of the photoresist unexposed to the light is removed is referred to as a negative type photoresist.
In general, a limit of resolution R (nm) in the photolithography technique using a reduction exposure method is represented by
R=k1xc2x7xcex/(NA) 
wherein xcex is an optical wavelength (nm) used, NA is a numeral aperture of a projection optical system of a lens, and k1 is a constant depending on a resist process and image-forming condition.
As can be seen from the equation above, a possible way to improve the limit of resolution R, i.e. to obtain a microscopic pattern, is to make the values of k1 and xcex smaller and to make the value of NA larger. That is, the constant depending on the resist process may be made lower while the wavelength is shortened and the NA is made higher.
However, problems arise in that it is technically difficult to improve a light source and a lens, and that the shorter wavelength and the higher NA may make the depth of focus 6 of light (xcex4=k2 xc2x7xcex/ (NA)2) shallower, resulting in lower resolution on the contrary.
In order to expose the pattern of the photomask onto the photoresist with high resolution in such a lithography technique, exposure must be carried out in the state where the photoresist is adjusted to the best image-forming plane, i.e. the best focus plane, of the projection optical system within a range of the depth of focus. For that purpose, it is necessary to obtain, in some way, the position of the best focus plane, i.e. the best focus position, of the projection optical system.
An example of a conventional focus monitor for measuring the best focus position is a phase shift focus monitor developed by Brunner of IBM Corp. and sold by Benchmark Technologies, Inc. in the United States.
FIG. 19 illustrates a method of phase shift focus monitoring. Referring to FIG. 19, a phase shift mask 105 is used in the phase shift focus monitoring method. Phase shift mask 105 includes a transparent substrate 105a, a light-shielding film 105b having a predetermined pattern, and a phase shifter 105c formed on the predetermined pattern.
Specifically, as shown in FIG. 20, phase shift mask 105 has a pattern in which a narrow light-shielding pattern is arranged between substantially wide transmitting portions 105d and 105e. It is noted that no phase shifter 105c is arranged at transmitting portion 105d, whereas phase shifter 105c is arranged at transmitting portion 105e. 
In the phase shift focus monitoring method, light is directed onto phase shift mask 105. At that moment, phase shifter 105c is configured such that the phase of the transmission light is shifted 90xc2x0, and thus, when the light transmitted through transmitting portion 105e is advanced compared to the light transmitted through transmitting portion 105b by the optical path difference of xc2xc xcex, {fraction (5/4)} xcex, . . . , or when it is delayed by xc2xe xcex,{fraction (7/4)} xcex, . . . , the both light beams intensify each other. This allows the light transmitted through phase shift mask 105 to have an asymmetric intensity distribution with respect to the z axis (the optical axis). The light transmitted through phase shift mask 105 is condensed by projection lens 119a, 119b, and forms an image onto a photoresist 121b on a semiconductor substrate 121a. 
By the phase shift focus monitoring, an image is formed onto photoresist 121b in the state where the intensity distribution of diffracted light is asymmetric with respect to the z axis. Thus, as wafer 121 is moved in the z direction, a pattern image on wafer 121 is moved in the direction perpendicular to the z axis which is in the lengthwise direction in the drawing (the x-y direction, i.e. the crosswise direction in the drawing). Measurement of the amount of shift of the pattern image in the x-y direction enables measurement of a position in the z direction, i.e. measurement of a focus.
Another example of the focus monitoring method other than the phase shift focus monitoring is a method disclosed in Japanese Patent Laying-Open No. 6-120116. In this method, first, a predetermined pattern on the surface of a photomask is illuminated with exposure light having a main light beam at the first angle of inclination, to expose the first image of the predetermined pattern on a photosensitive substrate. Subsequently, the predetermined pattern is illuminated by exposure light having a main light beam at the second angle of inclination different from the first angle of inclination, to expose the photosensitive substrate to the second image of the predetermined pattern. The distance between the exposed first and second images is measured, and the relation between the measured distance and a defocused amount is used to obtain the distance from the position of the photosensitive substrate to the best focus plane.
In this method, a photomask 205 having a configuration as shown in FIG. 21 is used for illumination of the predetermined pattern on the photomask surface at the first angle of inclination or the second angle of inclination.
Referring to FIG. 21, photomask 205 includes a transparent substrate 205a, position measurement marks 205b1, 205b2 formed on the front surface of transparent substrate 205a, a diffraction grating pattern 205c formed on the rear surface of transparent substrate 205a. This means that the exposure light entered into photomask 205 is diffracted at diffraction grating pattern 205c to illuminate position measurement mark 205b1 at the first angle of inclination and to illuminate position measurement mark 205b2 at the second angle of inclination.
However, the phase shift focus monitoring described above requires the use of a phase shift mask having a special structure as photomask 105. There was a problem in that such a photomask with the special structure increased the cost of the photomask.
In addition, the method disclosed in Japanese Patent Laying-Open No. 6-120116 requires formation of microscopic diffraction grating pattern 205c on the rear surface of the photomask, requiring a number of steps. Thus, there was a disadvantage in that the manufacturing cost of the mask was increased to a large degree.
Moreover, by the current mask fabricating techniques, it is extremely difficult to form patterns, while the relative positional relationship of the patterns on both sides of the mask substrate is maintained to be precise. If the patterns on the both sides are out of a desired relative positional relationship, position measurement marks 205b1, 205b2 are not illuminated with diffracted light at a desired angle, making it difficult to accurately measure focus.
Furthermore, there was a disadvantage in that only a portion of the rear surface of photomask 205 where diffraction grating pattern 205c exists must be illuminated with the exposure light, requiring the illumination range to be within a restricted portion.
An object of the present invention is to provide a focus monitoring method and a focus monitoring apparatus enabling inexpensive and highly-precise focus monitoring, and a method of semiconductor device, by eliminating the need for a special photomask.
According to the present invention, a focus monitoring method used for pattern formation of a semiconductor device is characterized in that light is directed onto a photomask by non-telecentric illumination obtained by controlling a shape of an opening of an illumination aperture, and such a characteristic is utilized to perform focus monitoring that a pattern image of the photomask formed by the illumination is moved in a direction perpendicular to an optical axis when an image-forming plane is moved in a direction of the optical axis.
In the focus monitoring method according to the present invention, non-telecentric illuminating light is directed onto the focus monitor to express a characteristic in that the image of the photomask pattern moves in the direction perpendicular to the optical axis when the image-forming plane is moved in the direction of the optical axis. The non-telecentric illumination can readily be obtained by controlling the shape of the illumination aperture, eliminating the need for the use of a special structure for the photomask. This allows an inexpensive and highly precise focus monitor.
Preferably, in the focus monitoring method, a mark pattern of a box-in-box-in-box type having an outer box pattern and an inner box pattern is transferred onto a photoresist, and a relative displacement of the outer box pattern and the inner box pattern transferred onto the photoresist is detected, to perform focus monitoring.
By using the mark pattern of the box-in-box type, defocus can be detected from the displacement within the image-forming plane.
Preferably, in the focus monitoring method, the non-telecentric illumination is used for exposure of at least one of the outer box pattern and the inner box pattern.
Thus, occurrence of defocus causes displacement of pattern images within the image-forming plane, so that defocus can be detected.
Preferably, in the focus monitoring method, the non-telecentric illumination is used for exposure of both of the outer box pattern and the inner box pattern. A first illumination aperture having an opening only on one side of a meridian plane set as a border is used at the time of exposure of the outer box pattern, and a second illumination aperture having an opening only on the other side of the meridian plane set as a border is used at the time of exposure of the inner box pattern.
Thus, defocus causes movements of the image of the outer box pattern and the image of the inner box pattern in the opposite directions, thereby enhancing detectivity.
Preferably, in the focus monitoring method, for the first illumination aperture, one of a circular illumination apperture stop, an annular illumination apperture stop and a quadruple illumination apperture stop, with an opening left only on one side of the meridian plane set as a border, is used. For the second illumination aperture, one of a circular illumination apperture stop, an annular illumination apperture stop and a quadruple illumination apperture stop, with an opening left only on the other side of the meridian plane set as a border, is used.
Thus, various diaphragms can be used as an illumination aperture.
The focus monitoring method preferably includes a first exposure step exposing the photoresist to one of the outer box pattern and the inner box pattern, a second exposure step exposing the photoresist to the other one of the outer box pattern and the inner box pattern, and a development step developing the photoresist after the first and second exposure steps.
Thus, development is performed after double exposure, to form the mark pattern of box-in-box type.
Preferably, the focus monitoring method includes a first exposure step exposing the photoresist to one of the outer box pattern and the inner box pattern, a first development step developing the photoresist after the first exposure step, a second exposure step exposing the photoresist to the other one of the outer box pattern and the inner box pattern, and a second development step developing the photoresist after the second exposure step.
Thus, by repeating exposure and development twice each, the mark pattern of the box-in-box type can be formed.
According to the present invention, a focus monitoring apparatus used for pattern formation of a semiconductor device, including an illumination optical system illuminating a photomask on which a pattern is formed with exposure light, and a projection optical system projecting an image of the pattern of the photomask onto a photosensitive body. The image of the pattern of the photomask, formed by directing non-telecentric illuminating light obtained by controlling a shape of an opening of an illumination aperture included in the illumination optical system onto the photomask, is configured to move in a direction perpendicular to an optical axis when an image-forming plane is moved in a direction of the optical axis.
The focus monitoring apparatus according to the present invention is configured to express the characteristic such that illumination of the photomask with non-telecentric illuminating light allows the pattern image to be moved in the direction perpendicular to the optical axis when the image-forming plane is moved in the direction of the optical axis. The non-telecentric illumination can readily be obtained by controlling the shape of the opening of the illumination aperture, eliminating the need for using a special structure for the photomask. This enables an inexpensive and highly precise focus monitor.
According to the present invention, a method of a semiconductor device uses any one of the focus monitoring methods described above.
This eliminates the need for a special photomask, allowing an inexpensive and highly precise focus monitoring, so that an inexpensive and highly precise pattern can be formed.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.